/*
 * Copyright 2002-2014 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.lee.util;

import java.util.Iterator;
import java.util.LinkedHashSet;
import java.util.NoSuchElementException;
import java.util.Set;

/**
 * Composite iterator that combines multiple other iterators, as registered via {@link #add(Iterator)}.
 *
 * <p>
 * This implementation maintains a linked set of iterators which are invoked in sequence until all iterators are
 * exhausted.
 *
 * @author Erwin Vervaet
 * @author Juergen Hoeller
 * @since 3.0
 */
public class CompositeIterator<E> implements Iterator<E> {

    private final Set<Iterator<E>> iterators = new LinkedHashSet<Iterator<E>>();

    private boolean inUse = false;

    /**
     * Add given iterator to this composite.
     * 
     * @param iterator iterator
     */
    public void add(Iterator<E> iterator) {
        Assert.state(!this.inUse, "You can no longer add iterators to a composite iterator that's already in use");
        if (this.iterators
                .contains(iterator)) { throw new IllegalArgumentException("You cannot add the same iterator twice"); }
        this.iterators.add(iterator);
    }

    @Override
    public boolean hasNext() {
        this.inUse = true;
        for (Iterator<E> iterator : this.iterators) {
            if (iterator.hasNext()) { return true; }
        }
        return false;
    }

    @Override
    public E next() {
        this.inUse = true;
        for (Iterator<E> iterator : this.iterators) {
            if (iterator.hasNext()) { return iterator.next(); }
        }
        throw new NoSuchElementException("All iterators exhausted");
    }

    @Override
    public void remove() {
        throw new UnsupportedOperationException("CompositeIterator does not support remove()");
    }

}
